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HCN2 Channels: A Permanent Open State and Conductance Changes

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Abstract

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in the membranes of heart and brain cells can conduct Na+ and K+ ions and activate between −30 and −120 mV. We express the α subunit of HCN2 channels in Xenopus laevis oocytes and are confronted with two unexpected problems. First, we observe a rise in membrane conductance at resting potential proportional to the amount of expression. On activation to hyperpolarizing potentials, the instantaneous conductance rises in proportion to the amount of activated current. CsCl reduces the observed effects. This can be explained by the expression in oocytes membranes of a fraction of permanently open HCN2 channels. Second, using TEVC technique, our data show a completely different behaviour in physiological solutions of heterogeneously expressed HCN2 currents from what is observed in wild-type currents in the absence of drugs. During pulse trains, we frequently observe (1) a fast and significant decline of the amplitude of HCN2 current during hyperpolarizing steps, (2) no recovery of this decline after a long period at resting membrane potential, (3) a different behaviour of the tail currents at depolarization with other and slower changes than during activation, (4) recovery of this decline in high K+/low Na+ bath solution. The decline of the HCN2 current in physiological conditions is caused by a reduction of the conductance of the HCN2 channel presumably caused by the mere presence of sodium in the channel, in competition with potassium ions and with a limitative effect on the channel conductance.

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Acknowledgments

We are very grateful to Dr. Steven A. Siegelbaum Ph.D. (Department of Pharmacology, Columbia University, New York) for his generous gift of mBCNG-2 cDNA encoding for the HCN2 channel and to Prof. Jan Tytgat (Laboratory of Toxicology, University of Leuven) for the preparation of the cRNA and the useful teaching of the techniques of surgical removal of ovarian tissue, preparation and injection of the oocytes. Dr. Alain Labro (Molecular biophysics, physiology and pharmacology, University of Antwerp) was so kind to provide us with cRNA from his constructed cDNA of HCN2 channel. We thank him for the many helpful discussions. We gratefully acknowledge the generous support of the Instituut Born-Bunge.

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  1. Laboratory for Electrophysiology, Department of Cardiology, Institute Born-Bunge, University of Antwerp, Universiteitsplein 1, 2610, Antwerp, Belgium

    François Pittoors & Pierre Paul Van Bogaert

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Correspondence to Pierre Paul Van Bogaert.

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Pittoors, F., Van Bogaert, P.P. HCN2 Channels: A Permanent Open State and Conductance Changes. J Membrane Biol 248, 67–81 (2015). https://doi.org/10.1007/s00232-014-9742-0

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